The present invention generally relates to improvements in sewing machines, in order to sew together workpiece parts having edges of equal or unequal length by means of a bead seam, and to improvements in so-called heavy-duty sewing machines intended to handle tough materials, such as leather. Especially, this invention relates to improvements in such sewing machines, which are intended to sew the edge of a plug to that of a vamp of a true moccasin shoe, while causing the longer edge of the vamp to be ruffled so as to match the edge length of the plug.
A `true moccasin` means a shoe, in which the vamp, a part of the upper, underlies the wearer's foot particularly in the toe area and is joined by a seam to a toe covering plug.
Conventionally, moccasin shoes are manufactured with a hand sewn seam; however, as described in the U.S. Pat. No. 3,530,808, filed June 2, 1969, this is a very expensive and tedious time consuming process requiring considerable skill of the operator. For this reason, many attempts have been made to develop a machine for processing moccasins which, in particular, is adapted to form a seam joining the outer edge of a plug to that of a vamp of a moccasin, while the much greater peripheral length of the vamp is puckered to coincide with the smaller peripheral length of the plug. It is a characteristical feature of the above-mentioned U.S. patent to provide a lockstitch sewing machine having two adjustable workpiece feeding devices, with means for imparting longitudinal work compressing motions to the vamp, whereby portions of the margin of the vamp are joined to the margin of the plug by stitches in puckered relationship. However, the range of application of the invention according to the aforesaid U.S. patent is limited to a so-called open edge ruffling seam. Since this type of seam is not waterproof, applicability is only limited to cheap moccasin footwear. Therefore, high quality moccasin shoes are fabricated with a bead seam formed by the protruding edge of the plug which is rolled over the edge of the vamp and stitched to the vamp and the plug.
In order to meet the requirements of the shoe industry for giving a machine manufactured moccasin shoe a hand sewn appearance, a new method using three sewing machines for producing moccasin shoes having the desired comfort factor and hand sewn appearance and achieving an increase of efficiency was published in the "DNZ-magazine No. 4/1977". This published method comprises the following steps:
(a) A first sewing machine automatically ruffles the peripheral length of the vamp, at which the amount of ruffling and the position of ruffling are adjustable by means of a preselector, or mechanically. Simultaneously, a tape is stitched to the vamp for fixing the ruffles. PA0 (b) Stitching of the plug to the vamp on a post-type sewing machine, and PA0 (c) final stitching with threads up to 0.6 mm and simultaneously rolling the protruding edge of the plug over the edge of the vamp for forming a bead seam.
According to a method published in an applicant's pamphlet, only two special purpose sewing machines for producing genuine moccasins and ruffling work on shoes are required. The first sewing machine is provided with a device for ruffling the vamp and simultaneously stitching the plug, with a metal plate (blade) located between the plug and the vamp for separating both parts for allowing ruffling. The second sewing machine stitches the bead seam with a coarse thread as the protruding edge of the plug is manually rolled over the edge of the vamp.
The aforesaid methods include three or two steps for joining the vamp and the plug. In the first method two auxiliary seams and in the second method one auxiliary seam are necessary for preparing the leather cuts prior to final joining by means of a lockstitch seam formed by coarse threads. Besides the fact that the auxiliary threads cannot be invisibly covered by this locksmith seam, stitching of three- respectively two-step-sequence causes an additional perforation in the material and a considerable weakness of the seam results.